Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety and for all purposes.
Angiogenesis is a regulated process involving the formation of new blood vessels. It plays an essential role in normal growth, embryonic development, wound healing, and other physiological processes (Yancopoulos et al. (2000) Nature, 407:242-8). Within the developing capillary, extracellular matrix (ECM) proteins serve as a structural scaffold for proliferating endothelial and tumor tissues and provide support for the growth of tumor cells. De novo angiogenesis is involved in several disease states including cancer, where the formation of new “embryonic-like” blood vessels (referred to as neovascularization herein) appear that differ from normal vasculature with regards to structure and function (Hanahan et al. (2000) Cell, 100:57-70). A number of in vivo and in vitro studies have demonstrated biological differences between normal and disease-associated vasculature using various model systems of angiogenesis, thereby raising the possibility of novel anti-angiogenic compounds that can selectively inhibit vessel formation of the embryonic-type, tumor-associated endothelial cells for therapy of neovascular disease. In light of these opportunities for therapy, an intense search for potential targets that can specifically inhibit tumor and other neovascular disease-associated endothelial or stromal (fibroblasts, pericytes, etc.) cell growth and function is ongoing.
In an attempt to identify such targets, strategies have been designed to identify cell surface antigens of tumor stroma as well as isolate specific proteins or RNA that are expressed in tumor stromal cells (Rettig et al. (1992) Proc. Natl. Acad. Sci. USA, 89:10832-6; St. Croix et al. (2000) Science, 289:1197-1202). These strategies have identified a cell surface protein that appears to be specifically expressed in tumor stromal cells referred to as endosialin (or tumor endothelial marker 1 (TEM1) or CD248).
Examination of gene expression patterns in normal and neoplastic tissue indicates upregulation of endosialin mRNA expression in tumor neovessels. (St Croix et al (2000) Science, 289:1197-1202). Similar endosialin expression levels were noted in human colorectal cancer (Rmali et al. (2005) World J. Gastroenterol., 11: 1283-1286), breast cancer tissues (Davies et al. (2004) Clin. Exp. Metastasis, 21:31-37), and histiocytomas (Dolznig et al. (2005) Cancer Immun., 5: 10). Human endosialin expression has been observed in highly invasive glioblastoma, anaplastic astrocytomas, and metastatic carcinomas, including melanomas (Brady et al. (2004) J. Neuropathol. Exp. Neurol., 63:1274-1283; Huber et al. (2006) J. Cutan. Pathol., 33:145-155).
The use of antibodies in immunohistochemistry studies have found robust expression of endosialin in a number of neovascular endothelial cells, fibroblasts and/or pericytes (Virgintino et al. (2007) Angiogenesis, 10:35-45) in malignant tissues, while expression in cell lines derived from embryonic-like endothelial cultures such as but not limited to HUVEC (Human Umbilical Vein Endothelial Cells) or HMVEC-(Neonatal Dermal Microvascular Endothelial Cells) is limited. Analysis of antibodies, polypeptides or non-protein ligands that can bind to endosialin have identified a subset of such molecules that can suppress the ability of endosialin to bind to its substrate and/or suppress intracellular activities leading to cell stasis or death.
Rettig et al. described monoclonal antibodies that recognize antigens on vessels within various cancer types (Rettig et al. (1992) Proc. Natl. Acad. Sci. USA, 89:10832-6). One of these was designated FB5 and was generated through immunization of mice with human embryonic fibroblasts. FB5 recognizes a ˜100 kDa protein on the surface of a neuroblastoma cell line, LA1-5s (U.S. Pat. No. 5,342,757). FB5 is a murine antibody (IgG1) that binds to endosialin and has been shown to recognize endothelial cells associated with a variety of different cancer types. Structural evaluation has classified endosialin as a C-type lectin-like, integral membrane protein, comprised of five globular extracellular domains (including a C-type lectin domain, one domain with similarity to the Sushi/ccp/scr pattern, and three EGF repeats). The protein also contains a mucin-like region, a transmembrane segment, and a short cytoplasmic tail. The protein appears to be a glycoprotein. Carbohydrate analysis shows that the endosialin core protein has an abundance of O-linked glycosylation (Christian et al. (2001) J. Biol. Chem., 276:48588-48595). Subsequent work combined the complementarity determining regions (CDRs) of the mouse FB5 into a human IgG1 backbone to create a humanized antibody that binds to vessels within malignant tissues as well as a subset of cells in HMVEC cultures.
Tem1 knockout mice develop normally and exhibit normal wound healing, suggesting that endosialin is not required for neovascularization during fetal development or wound repair. (Nanda et al. (2006) Proc. Natl. Acad. Sci. USA, 103:3351-3356). When colorectal cancer cells were implanted in the abdominal sites of Tem1 knockout mice, however, the loss of endosialin expression correlated with a reduction in tumor growth, invasion, and metastases as compared to parental animals. The absence of endosialin expression has been shown to reduce growth, invasion, and metastasis of human tumor xenografts in an endosialin knockout mouse. (Nanda et al. (2006) Proc. Natl. Acad. Sci. USA, 103:3351-3356). Additionally, lack of endosialin led to an increase in small immature blood vessels and decreased numbers of medium and large tumor vessels.
Neovascularization is associated with a number of disease states. In cancer it is believed that neovascularization is important to supply tumors with blood. In non-oncology cancer or malignant diseases such as retinopathy and macular degeneration, uncontrolled neovascularization causes loss of sight (Wilkinson-Berka (2004) Curr. Pharm. Des., 10:3331-48; Das and McGuire (2003) Prog. Retin. Eye Res., 22:721-48). Moreover, several reports have identified a role of neovascularization in inflammatory disease (Paleolog and Miotla (1998) Angiogenesis, 2(4):295-307). Methods to better understand molecular pathways in embryonic-like endothelial and precursor cells as well as endothelial-associated cells (pericytes, fibroblasts, etc.) associated with these disease states will lead to the development of novel drugs to treat these diseases. Conversely, neovascularization is associated with wound healing (Galiano et al. (2004) Am. J. Pathol., 164:1935-47). Identification of molecular pathways that promote vascularization for wound healing can offer the ability to identify drugs and factors that can promote these processes for enhancing wound treatment associated with trauma, burns and infection.
A difficult problem in effective antiangiogenic and proangiogenic therapy is the nondefined nature of biological processes of molecules and associated pathways that are important for activating cellular processes associated with neovascularization (Bagley et al. (2003) Cancer Res., 63:5866-73). The ability to identify and elucidate molecules and their function in regulating a given pathway can lead to the isolation of effective compounds that have stimulatory or inhibitory activity in neovascular-associated diseases such as cancer, inflammation, ocular disease, cardiovascular disease, and wound healing. The ability to isolate and study these compounds via molecular-based assays would provide further utility for evaluating their effects to specifically suppress or stimulate the normal biology of cells involved in neovascularization in contrast to adult-like endothelial cells associated with vessels in normal adult tissue (Asahara and Kawamoto (2004) Am. J. Physiol. Cell Physiol., 287:C572-9).